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Subject: [Bug target/101579] Suboptimal codegen for __builtin_shufflevector
Date: Wed, 28 Jul 2021 09:49:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101579

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Hongtao.liu from comment #4)
> I looks to me that middle end should be able to transform 64-byte vector
> shuffle to 32-byte vector shuffle when data flow analysis shows the upper
> part of the vector is never used.

That is not needed in this case.  The permutation is such that all indices =
for
the first half read from one half (in this case the first) and all indices =
for
the second half read from one half (in this case the first again), so it is
identical to a vector containing permutation of the first half with the fir=
st
half of the indices and permutation of the first half again with the second
half of the indices.=